A. Rationale The Virus Core is an essential resource for the needs of projects 1 and 2, which will benefit from a centralized Virus Core by: i) the established expertise with reverse genetics techniques which will facilitate rescue of recombinant influenza viruses that are described below and itemized in Table 1B, ii) the maintenance of influenza virus stocks that have been sequence-confirmed and assessed for quality by hemagglutination and plaque assays, and iii) reduced inter-experimental variation by consistent use of specific virus stock preparations. The Department of Microbiology, Mount Sinai School of Medicine, NY, NY is a pioneer in the application of reverse genetics and the development of recombinant viruses. The well equipped facilities, established procedures, and properly trained personnel provide a cost-effective Virus Core that will result in efficient production of wild-type and recombinant influenza virus stocks that will be essential for projects 1 and 2. Specifically, Dr. Megan Shaw (Co-PI of project 1) and Dr. Adolfo Garcia-Sastre (Co-PI of project 2) will directly benefit by their close proximity to and direct communication with the Virus Core. B, Specific functions of the Virus Core 1) Maintain working stocks of wild-type influenza viruses for use by projects 1 and 2. 2) Generate recombinant influenza viruses for use by projects 1 and 2.